By Keith Coffman

DENVER (Reuters) - Federal investigators said on Monday they have opened a criminal probe into the 2015 spill of some 3 million gallons (11 million liters) of toxic wastewater from a defunct Colorado gold mine that was triggered by a contractor with the Environmental Protection Agency (EPA).

At the urging of congressional leaders, the EPA's Office of Inspector General is investigating the rupture from the Gold King Mine above Silverton, Colorado, that fouled waterways in three states and Native American lands, the agency said in a statement.

"Based on requests from several members of the House and Senate, the OIG is conducting both a program evaluation and a criminal investigation of the Gold King Mine spill," the EPA said in a statement.

The OIG is an independent office that audits, investigates and evaluates the agency's activities, the EPA statement said.

Last August, a contractor hired by the EPA to slow seepage from the century-old stake breached a tunnel wall, unleashing a torrent of wastewater that had backed up behind the mountainside.

The orange-colored sludge, containing nearly 900,000 pounds (408, 233 kg) of heavy metals, poured into a creek that feeds the Animas and San Juan rivers and traversed into New Mexico, ultimately emptying into Lake Powell in Utah.

Two Republican members of the U.S. Senate's Committee on Indian Affairs, John McCain of Arizona and Chairman John Barrasso of Wyoming, sent a letter in May to Attorney General Loretta Lynch, urging her to launch a criminal probe into the spill.

On Monday, Barrasso applauded the probe.

"Tribal communities in the affected region have been devastated and the EPA has not taken responsibility for the mess it made," Barrasso said in a statement.  "I hope this investigation uncovers who is at fault and holds them responsible. We will be watching closely."

A report issued last fall by engineers with the U.S. Department of Reclamation concluded that the spill had been "preventable" and was caused by several missteps over several decades, including nearby mining operations and tunneling that the EPA overlooked when it opened a portal at the site.

  Jeffrey Lagda, a spokesman for the EPA's Inspector General, told Reuters that the probe has been ongoing for some time, and that investigators are working with prosecutors at the U.S. Attorney's Office in Denver.

The state of New Mexico has sued the EPA, the state of Colorado and two mining companies over the disaster.

(Reporting by Keith Coffman in Denver; Editing by Dan Whitcomb and Sandra Maler)